Astronomers just announced that Barnard’s Star, the closest single star to us and the second closest star system, has four confirmed planets orbiting it! And they’re wee.

I actually wrote about this for BAN Issue 781 last October, but at that time only one of the planets was confirmed, with the other three classified as candidates, since the data weren’t solid enough to say the planets existed for sure.

That’s now changed. Further observations using the MAROON-X spectrograph on the Very Large Telescope confirmed two of the candidates, and the third was confirmed when observations using the ESPRESSO spectrograph were added in [link to journal paper].

A spectrograph is an astronomical instrument that breaks up incoming light into individual colors, sometimes tens of thousands of very narrow slices of the spectrum. This opens up a lot of avenues to uncover characteristics of objects. For example, a star like Barnard’s Star has hydrogen and other elements in it that absorb light at very specific wavelengths, so a spectrum will show dark lines in it where that light was absorbed. That can tell you what’s in the star, how hot it is, and more.

In this case, it’s the “more” that’s critical. Planets orbit around stars, but planets also have mass. So, while the planet makes a big circle (or ellipse) around the star, its own gravity tugs on the star, which reacts by making a much smaller circle around the star and planet’s center of mass. This motion is incredibly small, but does create a Doppler shift in those absorption lines, so they shift very slightly in wavelength as the planet orbits. This radial velocity method can tell you the period of the planet (its year) and the mass of the planet, since a more massive planet will cause a larger shift than a lower mass one. 

That’s what the astronomers used to find the planets orbiting Barnard’s Star. Each planet made is own shift in the spectrum as it tugged on the star, and they all could be teased out of the data. The shifts were incredibly small, indicating the star was moving at velocities of 0.22 to 0.45 meters per second — that’s slower than walking speed! These measurements are incredibly precise, and it’s astonishing they could find them at all.

But those low values mean the planets are very low mass. The lowest mass one has only one-fifth Earth’s mass, making it the lowest mass planet ever found using this method. It’s only twice the mass of Mars! That is, that’s the lower limit to its mass; it might be heavier. I discuss how that works in the BAN article linked above.

Unfortunately, all four planets are too close to the star to be habitable; they’ll be too hot. But this discovery is still significant for a lot of reasons.

For one thing, Barnard’s Star is a red dwarf, a low-mass star cooler, smaller, and much fainter than the Sun. These are the most common kind of star in the galaxy — they make up about 75% of all stars, while stars like the Sun are only about 10% — and from what we’ve seen so far from observations and models of how planetary systems are formed, red dwarfs preferentially have smaller, rocky planets orbiting them (which are more like Earth, as opposed to gas giants like Jupiter). This strongly implies that the majority of Earth-sized (and, perhaps, Earth-like) planets orbit stars like them. That would also mean there are tens of billions of such planets in the galaxy, if not more. Tens of billions.

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